The present application relates to measurements of formation properties based on detection and analysis of neutron-induced gamma-ray emission and/or neutrons in the formation.
Forward modeling is useful for interpretation of measurements in non-trivial downhole logging geometries such as:                high-angle/horizontal wells through formation layer transitions;        thinly bedded formations,        intermediate invasion, and        various combinations of the above.These geometries share a common feature that a measurement's zone of sensitivity can encompass regions with different properties, causing the overall measurement at a single point to be an unknown combination of the constituent regions.        
For downhole nuclear logging measurements, the Monte Carlo method of modeling is typically used due to its accuracy. However, the Monte Carlo method of modeling is computationally intensive, making it unsuitable for fast modeling and interpretation of measurements in the non-trivial downhole logging geometries described above.
Forward models have been developed for several downhole nuclear measurements, including the gamma-gamma density, neutron porosity, and thermal neutron cross section (Sigma) measurements. Such forward models can be less accurate or physically complete than a full Monte Carlo simulation, but are typically much less computationally intensive and much faster to run, particularly for measurements in the non-trivial downhole logging geometries described above. However, forward models have not been developed for the measurement of formation density based on detection of neutron-induced gamma-rays from the formation.